This invention relates to stabilized polymerizable compositions comprising compounds containing moieties capable of free radical polymerization and organoborane initiators capable of initiating free radical polymerization and adhesives based on such compositions. In another embodiment the invention relates to a method of polymerizing compounds containing moieties capable of free radical polymerization and to methods of bonding substrates using such compositions.
Low surface energy olefins such as polyethylene, polypropylene and polytetrafluroethylene have a variety of attractive properties in a variety of uses, such as toys, automobile parts, furniture applications and the like. Because of the low surface energy of these plastic materials, it is very difficult to find adhesive compositions that bond to these materials. The commercially available adhesives that are used for these plastics require time consuming or extensive pretreatment of the surface before the adhesive will bond to the surface. Such pretreatments include corona treatment, flame treatment, the application of primers, and the like. The requirement for extensive pretreatment of the surface results in significant limitations to the designers of automobile components, toys, furniture and the like.
A series of patents issued to Skoultchi (U.S. Pat. Nos. 5,106,928; 5,143,884; 5,286,821; 5,310,835 and 5,376,746 (all incorporated herein by reference)) disclose a two-part initiator system that is useful in acrylic adhesive compositions. The first part of the two-part system includes a stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde. The organoborane compound of the complex has three ligands, which can be selected from C1-10 alkyl groups or phenyl groups. The adhesive compositions are disclosed to be useful in structural and semi-structural adhesive applications, such as speaker magnets, metal to metal bonding, automotive glass to metal bonding, glass to glass bonding, circuit board component bonding, bonding select plastics to metal, glass to wood, etc. and for electric motor magnets.
Zharov, et al. discloses in a series of US patents (U.S. Pat. No. 5,539,070; U.S. Pat. No. 5,690,780; and U.S. Pat. No. 5,691,065 (all incorporated herein by reference)) polymerizable acrylic compositions which are particularly useful as adhesives wherein organoborane amine complexes are used to initiate cure. It is disclosed that these complexes are good for initiating polymerization of an adhesive that bonds to low surface energy substrates.
Pocius in a series of patents (U.S. Pat. No. 5,616,796; U.S. Pat. No. 5,621,143; U.S. Pat. No. 5,681,910; U.S. Pat. No. 5,686,544; U.S. Pat. No. 5,718,977; and U.S. Pat. No. 5,795,657 (all incorporated herein by reference)) disclose amine organoborane complexes with a variety of amines such as polyoxyalkylene polyamines and polyamines which are the reaction product of diprimary amines and compound having at least two groups which react with a primary amine. Pocius (U.S. Pat. No. 5,686,544) discloses a composition comprising an organoborane polyamine complex, polyol and an isocyanate decomplexing agent.
Kendall et al., U.S. Pat. No. 6,630,555 and Kneafsey et al., U.S. Patent Publications 2003/0226472 and 2004/0068067 disclose quaternary boron salts which may be used as initiators for the polymerization in adhesive compositions. The adhesive compositions may be used for bonding low surface energy substrates.
Many of the initiators disclosed in the prior art are not stable in compositions containing olefinic unsaturation at, or near, ambient temperatures and induce polymerization at, or near, ambient temperature with time. This instability at or near, ambient temperature can result in polymerization before desired and can result in compositions that are unsuitable for the desired use. Further, the compositions generally are two part compositions in which one side is a resin side and the other side is a hardener. One side (hardener) contains the organoborane compound and the other contains the decomplexing agent. In most cases the volume ratio of the two parts is significantly different, that is greater, than 4:1, often up to about or at 10:1. The problem is that most commercial equipment designed to dispense two part compositions utilizes ratios of 4:1 or less. In order to get these compositions to work in such equipment, resin or nonreactive ingredients are added to one side or the other to get a suitable volumetric ratio. The problem is that if resin is added to the side containing the organoborane compound, the mixture may be unstable and could begin to cure at ambient temperatures and render the composition useless. If excessive inert materials are added to the hardener side, the inert ingredient can act as a plasticizer or create a weak continuous phase and negatively impact the properties of the polymerized composition.
Some of the references discussed above disclose the use of phenolic compounds, such as hydroquinones to stabilize the compositions against undesired polymerization. See Pocius U.S. Pat. No. 5,684,102 at column 18, lines 45-53; Pocius U.S. Pat. No. 5,861,910 at column 13 lines 17 to 24. Jennes U.S. Pat. No. 3,236,823 discloses the use of hydroquinones, phenathiazine or t-butyl pyrocatechol as stabilizers in alkylborane initiated acrylate systems.
Therefore, there is a need for adhesive systems which are capable of bonding to low surface energy substrates, and initiator systems which facilitate such bonding. What are further needed are polymer compositions and adhesive systems which are thermally stable at, or near, ambient temperatures and which will undergo polymerization when the user desires. Also needed are adhesive compositions which are capable of bonding to low surface energy substrates, and bonding low surface energy substrates to other substrates, without the need for extensive or costly pretreatment. Further compositions that can be used in existing commercial equipment at mix ratios of 4:1 or less are needed. Compositions that have stability, strength and adhesion at elevated temperatures are also desired.